This invention is derived from the familiar spray tower or spray chamber type of heat or mass exchanger. In these exchangers, an overhead array of nozzles injects a falling cloud or spray of liquid droplets into either a rising column or a cross-flow of gas.
The most common gas and liquid combination with which such an exchanger is used is air and water. Heat or water vapor is exchanged, or the water is aerated or degassed.
There are several problems that limit the performance of such an exchanger. One problem, which relates to the droplet nozzles, results from a natural hysteresis that requires a greater pressure to initiate a flow of liquid through a nozzle than is needed to sustain the flow once it has started This hysteresis is a result of the siphoning effect of the stream of liquid flowing from an active nozzle. Because of this hysteresis effect, it is difficult, when operating with a low pressure head, to activate all the nozzles of the exchanger, even if they are closely matched as to physical characteristics.
One method for distributing the flow evenly among all the nozzles is to constrict the nozzles so that they exhibit a significant dynamic back pressure. Though effective, this strategy imposes an energy penalty because of the greater pressure required for operation of the apparatus. Also, the narrow nozzle passages are especially vulnerable to clogging from scale and particulate matter build-up.
A second problem that limits the performance of a conventional spray tower is the fact that liquid droplets inevitably follow irregular paths as they fall. As a result, some droplets bump into laterally adjacent droplets and merge with them, while other droplets fall more quickly than, and plow into and merge with, more slowly moving droplets.
Another problem arises from the fact that for the most effective operation of a spray tower it is best for certain characteristics (such as temperature, or concentration of a give dissolved or suspended material) of the gas and liquid spray that make up the mixture in the spray tower to proceed progressively from top to bottom of the tower. This progression in various characteristics of the rising gas and the falling liquid spray tends to be destroyed by too thorough mixing of the gas and spray. Unless the ratio of the diameter of the tower to its length is very small (as with a tall, narrow spray tower) the gas will be quite thoroughly mixed with the spray, the relevant characteristics of the gas and liquid, respectively, will then be averaged out rather than varying progressively from the top to the bottom of the spray tower, and as a result full advantage of countercurrent flow can not be taken. However, the diameter/length ratio can not be made too small, since then the spray would quickly reach the walls of the tower and no longer constitute a spray of suspended droplets.
Finally, the tendency for the entrainment of liquid by the exiting gas is considerable, and mist eliminators are almost always necessary at the exit of a conventional spray tower.